Help! Trouble uploading large 1G file from Posterous

Hello - We are having a hard time importing our Posterous blog to WordPress. It is a large 1G file. We are able to download it via zip to our computer but when we try to open the file itself, it is corrupt. We are getting no help at all from Posterous and I am so scared we are going to lose all of our blog!!
It also states that WP can only import up to 15MB, is there a way to import a larger file if we can somehow figure out how to open the one we have?

You'll first need to unpack the .zip file. It includes a file named wordpress_export_1.xml. Although the other files serve as a backup of all of your content that you should consider saving for posterity, the xml file is what you’ll use to import into WordPress.com.